(a) Field of the Invention
This invention relates to a rubber dispersed copolymer resin having excellent heat resistance, impact strength and moldability. The resin of this invention has utility as a molding material, for example, for various parts in electric appliances, electronic equipment, cars, etc.
(b) Description of the Prior Art
Numerous attempts have heretofore been made to improve the heat resistance of vinyl polymers by copolymerization. As common tendency, there was such a problem that the impact resistance was lowered when the heat resistance was improved. With a view toward overcoming such a problem, there have been proposed resin compositions in each of which a copolymer and graft copolymer of an N-substituted maleimide and a vinyl monomer are mixed under specific conditions (for example, Japanese Patent Laid-Open Nos. 129043/1983, 206657/1983, etc.). In such compositions, the property balance between their heat resistance and Izod impact strength is maintained better than that in conventional resins (ABS) which are each composed of a copolymer and graft copolymer of a styrene-type monomer and an acrylonitrile-type monomer. However, there is still a room for further improvement in such conventional resins, especially, in connection with their maximum permissible impact strength.
On the other hand, it has also been proposed in U.S. Pat. No. 3,721,724 to polymerize a styrene-type monomer, acrylonitrile-type monomer and maleimide-type monomer in the presence of 1-20 wt. % pf a conjugated diolefin elastomer as a polymerization process for a copolymer having high impact strength and softening point. Although copolymers produced by this process had high Izod impact strength and high softening points, they were not satisfactory in maximum permissible impact strength.
Further, U.S. Pat. No. 3,652,726 discloses certain graft polymers. This specification is however silent about an improvement to the maximum permissible impact strength of a polymer by special grafting or occluding.
These days, heat resistant resins are used as molded parts of large dimensions and/or complex configurations in the fields of electric appliances, electronic equipment and cars. There is thus a strong demand for improvements to such resins in their moldability along with their heat resistance and maximum permissible impact strength. The term "maximum permissible impact strength" as used herein means resistance to impacts which are to be applied to a molded article in the course of its actual use due to its drop or application of impacts thereto. Especially, large molded articles having high heat resistance are weakest to impacts at areas where their wall thicknesses change or which are close to corners. There has thus been a demand for their improvement.
The maximum permissible impact strength of a resin does not correspond to the Izod impact strength of the same resin. It depends on the falling weight impact strength of a molded article at the above-mentioned areas, rather than its Izod impact strength. In connection with moldability, the flowability of a resin upon its molding generally tends to decrease when the resin is copolymerized with a heat resistance imparting monomer, the amount of rubbery component is increased and/or the molecular weight of the copolymer is increased so as to improve the heat resistance and maximum permissible impact strength of the resin. Serious problems in the above case were encountered because the molding cycle became longer, when the molding was done at a higher temperature and sometimes, even if the molding pressure was increased, the resin was not filled sufficiently in a mold and the molding operation itself was no longer feasible.